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2 | |
2 | |
3 | Guard - safe cleanup blocks |
3 | Guard - safe cleanup blocks |
4 | |
4 | |
5 | =head1 SYNOPSIS |
5 | =head1 SYNOPSIS |
6 | |
6 | |
7 | use Guard; |
7 | use Guard; |
8 | |
8 | |
9 | # temporarily chdir to "/etc" directory, but make sure |
9 | # temporarily chdir to "/etc" directory, but make sure |
10 | # to go back to "/" no matter how myfun exits: |
10 | # to go back to "/" no matter how myfun exits: |
11 | sub dosomething { |
11 | sub myfun { |
12 | scope_guard { chdir "/" }; |
12 | scope_guard { chdir "/" }; |
13 | chdir "/etc"; |
13 | chdir "/etc"; |
14 | |
14 | |
15 | call_function_that_might_die_or_other_fun_stuff; |
15 | call_function_that_might_die_or_other_fun_stuff; |
16 | } |
16 | } |
17 | |
17 | |
18 | =head1 DESCRIPTION |
18 | =head1 DESCRIPTION |
19 | |
19 | |
20 | This module implements so-called "guards". A guard is something (usually |
20 | This module implements so-called "guards". A guard is something (usually |
21 | an object) that "guards" a resource, ensuring that it is cleaned up when |
21 | an object) that "guards" a resource, ensuring that it is cleaned up when |
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52 | =item scope_guard BLOCK |
52 | =item scope_guard BLOCK |
53 | |
53 | |
54 | Registers a block that is executed when the current scope (block, |
54 | Registers a block that is executed when the current scope (block, |
55 | function, method, eval etc.) is exited. |
55 | function, method, eval etc.) is exited. |
56 | |
56 | |
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57 | See the EXCEPTIONS section for an explanation of how exceptions |
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58 | (i.e. C<die>) are handled inside guard blocks. |
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59 | |
57 | The description below sounds a bit complicated, but that's just because |
60 | The description below sounds a bit complicated, but that's just because |
58 | C<scope_guard> tries to get even corner cases "right": the goal is to |
61 | C<scope_guard> tries to get even corner cases "right": the goal is to |
59 | provide you with a rock solid clean up tool. |
62 | provide you with a rock solid clean up tool. |
60 | |
63 | |
61 | This is similar to this code fragment: |
64 | The behaviour is similar to this code fragment: |
62 | |
65 | |
63 | eval ... code following scope_guard ... |
66 | eval ... code following scope_guard ... |
64 | { |
67 | { |
65 | local $@; |
68 | local $@; |
66 | eval BLOCK; |
69 | eval BLOCK; |
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70 | |
73 | |
71 | Except it is much faster, and the whole thing gets executed even when the |
74 | Except it is much faster, and the whole thing gets executed even when the |
72 | BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
75 | BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. |
73 | |
76 | |
74 | If multiple BLOCKs are registered to the same scope, they will be executed |
77 | If multiple BLOCKs are registered to the same scope, they will be executed |
75 | in reverse order. Stuff like C<local> is managed via the same mechanism, |
78 | in reverse order. Other scope-related things such as C<local> are managed |
76 | so variables C<local>ised after calling C<scope_guard> will be restored |
79 | via the same mechanism, so variables C<local>ised I<after> calling |
77 | when the guard runs. |
80 | C<scope_guard> will be restored when the guard runs. |
78 | |
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79 | See B<EXCEPTIONS>, below, for an explanation of exception handling |
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80 | (C<die>) within guard blocks. |
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81 | |
81 | |
82 | Example: temporarily change the timezone for the current process, |
82 | Example: temporarily change the timezone for the current process, |
83 | ensuring it will be reset when the C<if> scope is exited: |
83 | ensuring it will be reset when the C<if> scope is exited: |
84 | |
84 | |
85 | use Guard; |
85 | use Guard; |
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103 | when the BLOCK gets executed: when the last reference to the object gets |
103 | when the BLOCK gets executed: when the last reference to the object gets |
104 | destroyed, the BLOCK gets executed as with C<scope_guard>. |
104 | destroyed, the BLOCK gets executed as with C<scope_guard>. |
105 | |
105 | |
106 | The returned object can be copied as many times as you want. |
106 | The returned object can be copied as many times as you want. |
107 | |
107 | |
108 | See B<EXCEPTIONS>, below, for an explanation of exception handling |
108 | See the EXCEPTIONS section for an explanation of how exceptions |
109 | (C<die>) within guard blocks. |
109 | (i.e. C<die>) are handled inside guard blocks. |
110 | |
110 | |
111 | Example: acquire a Coro::Semaphore for a second by registering a |
111 | Example: acquire a Coro::Semaphore for a second by registering a |
112 | timer. The timer callback references the guard used to unlock it again. |
112 | timer. The timer callback references the guard used to unlock it |
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113 | again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard> |
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114 | method that does this already): |
113 | |
115 | |
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116 | use Guard; |
114 | use AnyEvent; |
117 | use AnyEvent; |
115 | use Coro::Semaphore; |
118 | use Coro::Semaphore; |
116 | |
119 | |
117 | my $sem = new Coro::Semaphore; |
120 | my $sem = new Coro::Semaphore; |
118 | |
121 | |
119 | sub lock_1s { |
122 | sub lock_for_a_second { |
120 | $sem->down; |
123 | $sem->down; |
121 | my $guard = guard { $sem->up }; |
124 | my $guard = guard { $sem->up }; |
122 | |
125 | |
123 | my $timer; |
126 | my $timer; |
124 | $timer = AnyEvent->timer (after => 1, sub { |
127 | $timer = AnyEvent->timer (after => 1, sub { |
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130 | |
133 | |
131 | The advantage of doing this with a guard instead of simply calling C<< |
134 | The advantage of doing this with a guard instead of simply calling C<< |
132 | $sem->down >> in the callback is that you can opt not to create the timer, |
135 | $sem->down >> in the callback is that you can opt not to create the timer, |
133 | or your code can throw an exception before it can create the timer, or you |
136 | or your code can throw an exception before it can create the timer, or you |
134 | can create multiple timers or other event watchers and only when the last |
137 | can create multiple timers or other event watchers and only when the last |
135 | one gets executed will the lock be unlocked. |
138 | one gets executed will the lock be unlocked. Using the C<guard>, you do |
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139 | not have to worry about catching all the places where you have to unlock |
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140 | the semaphore. |
136 | |
141 | |
137 | =item Guard::cancel $guard |
142 | =item Guard::cancel $guard |
138 | |
143 | |
139 | Calling this function will "disable" the guard object returned by the |
144 | Calling this function will "disable" the guard object returned by the |
140 | C<guard> function, i.e. it will free the BLOCK originally passed to |
145 | C<guard> function, i.e. it will free the BLOCK originally passed to |
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164 | Therefore, whenever a guard block throws an exception, it will be caught, |
169 | Therefore, whenever a guard block throws an exception, it will be caught, |
165 | and this module will call the code reference stored in C<$Guard::DIED> |
170 | and this module will call the code reference stored in C<$Guard::DIED> |
166 | (with C<$@> set to the actual exception), which is similar to how most |
171 | (with C<$@> set to the actual exception), which is similar to how most |
167 | event loops handle this case. |
172 | event loops handle this case. |
168 | |
173 | |
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174 | The default for C<$Guard::DIED> is to call C<warn "$@">. |
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175 | |
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176 | The C<$@> variable will be restored to its value before the guard call in |
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177 | all cases, so guards will not disturb C<$@> in any way. |
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178 | |
169 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
179 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
170 | not guaranteed, but right now, the exception will simply be ignored). |
180 | not guaranteed, but right now, the exception will simply be ignored). |
171 | |
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172 | The default for C<$Guard::DIED> is to call C<warn "$@">. |
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173 | |
181 | |
174 | =head1 AUTHOR |
182 | =head1 AUTHOR |
175 | |
183 | |
176 | Marc Lehmann <schmorp@schmorp.de> |
184 | Marc Lehmann <schmorp@schmorp.de> |
177 | http://home.schmorp.de/ |
185 | http://home.schmorp.de/ |
178 | |
186 | |
179 | =head1 THANKS |
187 | =head1 THANKS |
180 | |
188 | |
181 | To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to |
189 | Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED> |
182 | the problem of exceptions. |
190 | solution to the problem of exceptions. |
183 | |
191 | |
184 | =cut |
192 | =cut |
185 | |
193 | |